1. Field of the Invention
The present invention relates generally to telemetry systems for measurement while drilling ("MWD") technology and more particularly relates to a nonlinear equalizer to reduce nonlinear signal distortion present in a transmission channel such as the mud column of an MWD telemetry system, thereby transmitting data with a relatively increased data rate and with improved accuracy.
2. Description of Related Art
The chief traditional method for obtaining information about subsurface formations surrounding an oil well is wireline logging. Generally, in wireline logging a sensitive measuring instrument is lowered down a wellbore, and measurements are made at different depths of the well. The measuring instrument may take various forms as required, for example, to perform electric logs, nuclear logs, or formation testing logs, etc. Electrical logs are typically used to locate hydrocarbon reserves. In contrast, nuclear logs are employed to determine the volume of hydrocarbons in the reserves, typically by determining the porosity of the materials in reserves identified by the electrical logs. In addition to electric and nuclear logs, formation pressure testing logs are often used to determine the moveability of the reserves, chiefly by determining the reserves' pressure and permeability.
One disadvantage of wireline logging is that its procedures are time consuming. For instance, wireline logging cannot be started until the well is drilled and the drilling equipment is completely removed from the well. To overcome this limitation, geophysicists have been researching and developing techniques such as "measurement while drilling" and "logging while drilling" techniques. With logging while drilling, downhole logging tools are used to transmit logging data such as gamma ray and resistivity measurements upward through the mud column while the drilling string is deployed downhole. With measurement while drilling, data concerning different aspects of the drilling process is measured and transmitted to the surface while the drilling string is downhole. Such data, for example, may involve directional drilling data, mud flow data. These technologies are often generically referred to as "MWD" operations.
In MWD operations, measurements are made downhole while drilling, and representative data signals are transmitted uphole through the drilling mud ("mud column") in the well. A subassembly, often positioned in the drilling string near the drill bit, may be used to carry the measuring equipment. Data is often transmitted to the surface during pauses between drilling activity, such as when additional sections of pipe are being added to the string.
In an exemplary MWD arrangement (FIG. 1 ), a drill string 100 is present in a wellbore 102. As the bit 104 drills deeper and deeper into the earth, more and more of the drill string 100 is lowered into the created wellbore 102. To aid in cooling the bit 104 and to assist in removing cuttings generated by the bit 104, the wellbore 102 is filled with drilling mud 106, which is circulated by a mud pump 108. The mud pump 108 injects drilling mud downward through a hollow, central conduit 110 interior to the drilling string. The mud eventually advances through passages (not shown) in the bit 104, travels upward in an annulus 112 between the drill string 100 and the wellbore 102, and returns to the mud pump 108. Mud returning to the pump 108 may be processed to remove cuttings and other undesirable materials.
Downhole characteristics of the mud flow, formation, temperature, and the like are analyzed by one or more measurement tools (not shown), which translate these measurements into digital signals. For instance, binary signals (i.e., logic 1 and logic 0 values) may be used. Then, a mud pulser 114 receives the digital signals, and selectively interrupts the flow of mud through the conduit 110 (i.e., "pulses") in response to the digital signals.
In the illustrated embodiment, the mud pulser 114 includes a poppet valve, which includes a rounded piston 116 and a mud valve 118 that comprises a narrowed aperture sized to sealingly receive the piston 116. Hence, by reciprocating upward and downward, the piston 116 is able to selectively restrict the flow of mud through the mud valve 118. At the surface, a pressure transducer (not shown) is used to detect and measure pressure wave fluctuations created in the mud column by the mud pulser 114, to assist in decoding the signals transmitted by the mud pulser 114.
Another embodiment of mud pulser is the "mud siren", which continuously phase modulates a sinusoidal wave generated by a rotary valve. The phase modulation is achieved by changing the speed of the motor rotating the valve.
Conventional MWD systems provide data with a reliability level, and at a data rate, that are insufficient for some users or for some applications. These limitations can become particularly problematic as MWD technology is relied upon to handle increasing quantities and varieties of downhole data.
Some of MWD's technical limitations result from the data signal (the "mud pulse waveform") being distorted by the mud column during transmission to the surface. Many MWD telemetry systems operate at 1-2 bits per second, and employ simple binary mud-pulse waveforms. However, the telemetry receivers of these systems typically fail to correct for distortion introduced by the mud column.
Some systems, in attempts to increase the accuracy and rate of data transmission of MWD telemetry systems, have utilized relatively complex telemetry waveforms and receiver techniques from other telemetry system technologies. For example, MWD telemetry receivers have been proposed which employ an adaptive linear channel equalizer such as a transversal filter equalizer ("TFE") or a decision feedback equalizer ("DFE") to compensate for signal distortion caused by the mud transmission channel.
Although these linear equalizers are helpful in some situations, in many applications they can fail to adequately correct channel distortion when complex data signals are used, since MWD systems inherently include a number of nonlinear characteristics. In this context, the "nonlinearity" relates, for example, to the effects that pulsing of the mud transmission channel at a single frequency may provide a multi-frequency output at the surface, and that the frequency of the output signal may vary depending upon the amplitude of the input signal.
One factor that contributes to the nonlinearities of MWD systems is the nonlinearity of the mud pulse transmitter. For example, the mud pulse transmitter's ability to output the desired mud pressure signal shape may be degraded due to limitations in the fluid velocity rate. Also, it may be mechanically difficult for the mud pulser 114 to overcome the significant downward pressure exerted by the mud column. Another factor contributing to MWD systems' nonlinearities is the nonlinearity of mud columns themselves, due to their non-homogeneous fluid characteristics. Also, signals produced by mud pulsers are nonlinear: signals are produced by changes in pressure created by the piston 116, which is a function of the area of the conduit 110 and the distance that the piston 116 reciprocates. Also, the characteristics of the mud pulser 114 itself may change over time in a nonlinear fashion, due to wear.
Therefore, because most MWD telemetry receivers do not adequately compensate for nonlinearities in the mud column, many MWD telemetry systems are unable to employ more complex signal modulation techniques, and hence may not have a sufficiently high reliability and data transmission rate to optimally satisfy telemetry demands.